In general, a color picture tube is constructed to display a color image by scanning a phosphor screen in horizontal and vertical directions by a plurality of electron beams emitted from an electron gun.
An example of the electron gun applied to the color picture tube is an inline electron gun, which emits three electron beams in line: a center beam and a pair of side beams on both sides of the center beam, all traveling on the same horizontal plane. A main lens portion of the electron gun is constituted by grids. The center axes of side beam passage holes, through which side beams pass in a grid on the higher voltage side of all the grids, are decentered to outer sides than those of a grid on the lower voltage side. In other words, the center axes of side beam passage holes in a grid on the higher voltage side are located on outer portions, which apart from center beam, than those of a grid on the lower voltage side. As a result, the three electron beams are converged in a central portion of the screen. The three electron beams aligned in line can be self-converged in all the region of the screen, if a deflection field is pincushion-shaped in the horizontal direction, i.e., the inline direction in which the electron beams are aligned, and barrel-shaped in the vertical direction, i.e., the direction perpendicular to the inline direction.
Of the electron guns used in the color picture tube, a type of electron gun for improving the focus characteristic in all the region of the screen is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 64-38947. The electron gun is called an extended field type electron gun, which comprises a number of focusing grids, and in which part of an anode voltage is resistance-divided by a resistor arranged within a neck of the color picture tube, so that the divided voltages can be supplied to the grids, thereby forming a large-diameter main lens having a long focus by moderate potential distribution.
FIGS. 1A and 1B show an example of the extended field type electron gun.
As shown in FIG. 1A, the electron gun comprises three cathodes KB, KG and KR aligned in line, each incorporating a heater (not shown) for emitting electron beams. The electron gun further comprises a first grid 10, a second grid 20, a third grid 30, a fourth grid 40, a fifth grid 50, a plurality of intermediate electrodes 70 and 80, a sixth grid 60 and a convergence cup 90. These components are arranged in this order in the direction of travel of the electron beams and supported and fixed to each other by an insulating support (not shown).
A resistor 100 is provided near the electron gun, as shown in FIG. 1B. One end 110 of the resistor 100 is connected to the sixth grid 60, while the other end 120 is grounded. Intermediate points 130 and 140 are respectively connected to the intermediate electrodes 70 and 80. The end 110 of the resistor 100 is also connected to an operation voltage supplying device 131.
As shown in FIGS. 1A and 1B, the first grid 10 is a thin plate electrode having three beam passage holes of a small diameter to allow passage of electron beams. The second grid 20 is also a thin plate electrode having three beam passage holes of a small diameter to allow passage of electron beams.
The third grid 30 is formed of two cup electrodes 31 and 32, open ends of which are joined together. The cup electrode 31, on the side of the second grid, has three beam passage holes having a diameter slightly larger than that of the beam passage holes formed in the second grid 20. The cup electrode 32, on the side of the fourth grid, has three beam passage holes having a large diameter.
The fourth grid 40 is formed of two cup electrodes 41 and 42, open ends of which are joined together. Each of the cup electrodes 41 and 42 has three beam passage holes, having a large diameter. The fifth grid 50 is formed of a plurality of cup electrodes 51, 52, 53 and 54, each having three beam passage holes having a large diameter.
The intermediate electrodes 70 and 80 are thick plate electrodes, each having three beam passage holes of a large diameter. The sixth grid 60 is formed of two cup electrodes 61 and 62, open ends of which are joined together. Each electrode has three beam passage holes having a large diameter. The convergence cup 90 is fixed to the bottom of the cup electrode 62.
A DC voltage of about, for example, 100 to 150V and a modulation signal corresponding to an image superimposed thereon are applied to the three cathodes KB, KG and KR. The first grid 10 is grounded. The second and fourth grids 20 and 40 are connected to each other in the tube, and a DC voltage of about 600 to 800V is applied thereto.
The cathodes KB, KG and KR, the first grid 10 and the second grid 20 constitute a triode. The triode emits electron beams and forms a crossover.
The third and fifth grids 30 and 50 are connected to each other in the tube, and a focusing voltage of about 6 to 9 kv is applied thereto. An anode voltage of about 25 to 30 kV is applied to the sixth grid 60.
The second and third grids 20 and 30 form a prefocus lens, which preliminarily focuses the electron beams emitted from the triode.
The third, fourth and fifth grids 30, 40 and 50 form an auxiliary lens, which further preliminarily focuses the electron beams output from the prefocus lens.
A voltage corresponding to about 40% of the anode voltage is applied to the intermediate electrode 70 by the resistor 100 provided near the electron gun. A voltage corresponding to about 65% of the anode voltage is applied to the intermediate electrode 80. Thus, the voltages of the values substantially intermediate between the voltages, applied to the fifth and sixth grids 50 and 60, are applied to the intermediate electrodes 70 and 80.
In the above structure, the fifth grid 50, the intermediate electrodes 70 and 80 and the sixth grid 60 form a main lens, which finally focuses the electron beams on the screen. The main lens, having a main lens region extended by the intermediate electrodes 70 and 80, is called an extended field lens.
The side beam passage holes formed in the intermediate electrode 80 and the cup electrode 61 are decentered to outer sides from the center axes of the holes. Therefore, the side beams are deflected toward the center beam, with the result that the three electron beams are converged substantially at the center of the screen.
In the conventional electron gun of the extended field type, the main lens formed of the fifth grid 50, the intermediate electrodes 70 and 80 and the sixth grid 60 has a large diameter, so that the focusing performance in the all region of the screen is greatly improved. However, while the color picture tube is operating, the current tends to leak from the resistor 100 arranged within the neck. Since no measures to cope with the current leak are taken in the conventional electron gun, if the current leaks, the voltages applied to the intermediate electrodes 70 and 80 are unstable, resulting in change in the focusing characteristic of the main lens. If the focusing characteristic changes, a so-called convergence characteristic, for converging the three electron beams onto one point, also varies on the phosphor screen side.